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learn from my mistake...

WARNING, if your temp gauge seems faulty jumping to hot and back to normal while you know for sure that your car isnt hot, it is an easy fix, you need a new thermostat. It comes with a sensor attached. I figured that it wasnt that big of a problem and I would fix it next time I took it in. WRONG ANSWER! The sensor on the thermostat shorted out the ECM and usually burns the wiring harness. Yes thats right, 2 wires coming off the thermostat killed the brain of my car. If you see this problem in your car fix it immediately or your pockets will suffer the wrath of the BMW dealer cause thats only where you will find these parts. 1998 was the experimental year for separating the Engine and Trans wiring harness. I guess they figured that it didnt work well cause the 1999 540 wiring harness is identical to 1997. 1998 is the only year they separated them. My mechanic says its not a common problem but he has seen it before. the Engine ECM was the price of a used Honda, the wiring Harness was the same but I was lucky just the ECM went out. New thermostat was $260.

Re: learn from my mistake...

In the push for BMW to get models out the door and the focus on crash testing and now pedestrain safety the design of fail-safe and root cause effects seems to be a thing of the past for automotive electrical/systems design. Manufacturers are also always looking at ways to reducing their costs from my experience as well.

I know with my earlier BMWs (1992 vintage) I had to modify myself the wiring of the under dash buzzer circuit to eliminate/attenuate an annoying buzz coming from the peizo, probably due to leakage current from a marginal output transistor design, but I don't really know. In the case of variable resitive sensors you would think the designers would put in short-circuit protection in the controller outputs or introduce a series current limiting resistor and calibrate it out using the software, but it seems the automotive electronics world is not as well developed as the consumer hand-held electronics world, where design control and user safety is a real legal and financal risk and therefore designed to a certain standard and internal company standard.

It always amazes me how industries always seem to only hire people from within the same industry. The wealth of knowledge and experience can always benefit from learning from others, but it does not seem to be the case in most circles of industry.

BMWs are also not cheap to buy and fix, so forums like this really help keeping the cars alive and keep owners sane.

Close!

Your lesson is indeed a valuable one, although with a little twist from what you've reported. According to everything I know about the M62(and TU), the additional wires to the t-stat are not for a sensor but for a HEATER. This heater is energized by the ECU to cause it to open early and reduce the setpoint temperature. That's why this circuit is fused so highly (30A+) as opposed to a sensor.

If your temp gauge was jumping around, it probably still will, because the sensor is independent.

There's been reports here that many factory-installed heated t-stat housings have a casting flaw which must eventually eat through the insulation on the heater wires which pass through the housing. My dad melted those same wires and blew a bunch of fuses. I gave him an early-warning heads up but he mistakingly inspected the wrong wires.

At the time, this was the URL I sent him. Make letter substitutions to access it: http://www.bixxerfest.com/forums/showpost.php?p=4336370&postcount=1

My car has two devices threaded into the>>

t'stat housing: the heater, to the right, and the dual temp sensor, with two cables: one to the DME, and one to the instrument custer.

The DME sinks the current from the heating element, but the wires are quite small; I'm sure the element does not draw 30A! Besides, that 30A fuse supplies other circuits.

However, if the wires touch, the DME would have no current-limiting resistance from the heating element.

It might be prudent to install an in-line fuse in the leg that runs to the DME, 5A?

Where do you suppose the wires chafe? And why does that cause the temp gauge to jump around?
Ed in San Jose. BMW CCA member since 1987 (Nr. 62319). Golden Gate Chapter. '97 540i 6 speed. Build Date 3/97. Aspensilber over Aubergine leather.

T-stat heater draws ~1.5 amps IIRC

"Drive It Like You Stole It, A Sickness for Quickness"The Bottle Rocket "King of Spray" 2 Stage Nitrous OxideDare to be different and stand out among the crowd of me-to-cars!No guts, no glory!Tire smoke, not traction control!Zionsville all aluminum radiator to replace the 3 leak prone Nissens Radiators!

So did Joe M. over in Yurrip

"Drive It Like You Stole It, A Sickness for Quickness"The Bottle Rocket "King of Spray" 2 Stage Nitrous OxideDare to be different and stand out among the crowd of me-to-cars!No guts, no glory!Tire smoke, not traction control!Zionsville all aluminum radiator to replace the 3 leak prone Nissens Radiators!